1. Field of the Invention
The present invention relates to a transmission type element having first and second holograms that are Lippmann holograms and having wavelength selectivity and also to a transmission observation type anti-forgery volume hologram having wavelength selectivity that is particularly suitable for forgery prevention.
2. Description of the Related Technology
As general characteristics of holograms, it is known that Lippmann holograms (volume and reflection type holograms) have a high-angle selectivity and high-wavelength selectivity while transmission type holograms (volume type/surface relief type) give rise to color dispersion to a large extent. FIG. 14 is a schematic illustration of the spectral transmittance distribution of a reflection type hologram and that of a transmission type hologram. As illustrated in FIG. 14, reflection type holograms have high-wavelength selectivity whereas transmission type holograms have low-wavelength selectivity.
Holography is a technique of recording both the amplitude and the phase of a light into photosensitive material as interference fringe between reference and object light. While ordinary photography can provide only an image observed from a single angle of view, a hologram prepared by means of this technique can reproduce a three dimensional image obtained by observing an object from different angles of view. A hologram requires a sophisticated technique for preparation and also an apparatus that is sophisticated and costly. Therefore, it is generally difficult to forge or alter a hologram and hence attempts are being made to use holograms as anti-forgery means for certificates and securities, exploiting the difficulty of forgery of holograms.
While a sophisticated optical design technique and costly equipment are required to copy a hologram, it is possible to copy a hologram by contact copy method, which photosensitive material is stacked on master hologram, and light for recording enters from photosensitive material side in case of a reflection type hologram and from master hologram side in case of a transmission type hologram. For example, it has been proposed to prevent copying a hologram for the purpose of forgery by dividing a hologram into element regions and varying the angle of reference light for each region (see Patent Document 2: JP-A-08-123305).
An invention relating to an image combiner for using a reflection type hologram in the inside of an element and making light from an image display means and external light overlap each other has been disclosed (Patent Document 1: JP-A-2004-061731).
However, it is difficult to use the above-described arrangement for synthesis/separation of light. Light has to enter from the edge of light guide because of adapting total reflection, so area of incident light is limited by the area of edge of light guide, that results in decrease of total amount of light and difficulty in case of applications that needs wide area of incident light.
Additionally, when a transmission type hologram is used as an optical element, light other than the designed wavelength is also diffracted because of low wavelength selectivity, that gives rise to trouble when used for synthesis/separation of a light path in certain applications.
In view of the above-identified problems of the conventional technology, the object of the present invention is to provide a transmission type optical element having excellent wavelength selectivity and a transmission type anti-forgery volume hologram that is not costly and structurally simple but has excellent wavelength selectivity and is difficult to be forged.